U.S. patent number 7,247,822 [Application Number 10/772,641] was granted by the patent office on 2007-07-24 for carbon fiber heating element assembly and methods for making.
This patent grant is currently assigned to Methode Electronics, Inc.. Invention is credited to James J. Johnston.
United States Patent |
7,247,822 |
Johnston |
July 24, 2007 |
**Please see images for:
( Certificate of Correction ) ** |
Carbon fiber heating element assembly and methods for making
Abstract
An electrical resistance heating element has an axially
elongated flat carbon fiber tow, which includes a multiplicity of
continuous axially parallel carbon filaments. The tow is sandwiched
between two layers of polyester sheet material and bonded to only
one of the layers. The other of the layers overlies the tow in
direct contacting engagement with and unconnected relation to the
tow and is connected to longitudinally extending marginal portions
of the one layer along transversely opposite sides of the tow. The
heating element may be produced by a continuous forming
process.
Inventors: |
Johnston; James J. (St.
Petersburg, FL) |
Assignee: |
Methode Electronics, Inc.
(Chicago, IL)
|
Family
ID: |
34860785 |
Appl.
No.: |
10/772,641 |
Filed: |
February 5, 2004 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20050184051 A1 |
Aug 25, 2005 |
|
Current U.S.
Class: |
219/549; 219/213;
392/435 |
Current CPC
Class: |
H05B
3/145 (20130101) |
Current International
Class: |
H05B
3/34 (20060101) |
Field of
Search: |
;219/213,528,529,542,546,548,549,544,550,203 ;392/432,435
;338/210,212,254,255,306 ;343/702 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Campbell; Thor S.
Attorney, Agent or Firm: McCormick, Paulding & Huber
LLP
Claims
I claim:
1. A heating element assembly comprising; an electrical heating
element including an axially elongated substantially flat bundle
formed by a multiplicity of continuous axially extending carbon
fibers which transforms electrical energy applied thereto into heat
energy, said bundle having upper and lower surfaces including
generally flat upper and lower surface portions substantially
parallel to each other and a predetermined electrical resistance
per unit of axial length, and a dielectric sheath embracing said
bundle along its axial length, and including a lower layer having
an upper face bonded to said lower surface of said bundle and an
upper layer having a lower face disposed in overlying direct
contacting engagement and unconnected relation to said upper
surface of said bundle.
2. A heating element assembly as set forth in claim 1 wherein said
bundle comprises from several hundreds to several tens of thousands
of individual carbon fibers.
3. A heating element assembly as set forth in claim 1 wherein said
bundle comprises a carbon fiber tow having from 1 thousand to 50
thousand generally cylindrical carbon fibers each having a diameter
ranging from 6 to 10 microns.
4. A heating element assembly as set forth in claim 1 wherein said
bundle comprises a carbon fiber tow having 50 thousand generally
cylindrical fibers each having a 7 micron diameter.
5. A heating element assembly as set forth in claim 1 wherein said
sheath comprises a thermoplastic material.
6. A heating element assembly as set forth in claim 5 wherein said
thermoplastic material comprises a polyester.
7. A healing element assembly as set forth in claim 6 wherein said
polyester comprises MYLAR.
8. A heating element assembly as set forth in claim 1 where said
sheath comprises a thermosetting material.
9. A heating element assembly as set forth in claim 1 wherein said
thermosetting material comprises a polyimide.
10. A heating element assembly as set forth in claim 9 wherein said
polyimide comprises KAPTON.
11. A heating element assembly as set forth in claim 1 wherein said
upper and lower layers are formed by separate webs of dielectric
sheet material arranged in face-to-face relation to each other with
said bundle therebetween and said layers have marginal portions
extending outwardly in opposite transverse directions beyond
longitudinally extending side edges of said bundle and bonded
together and sealed in face-to-face relation to each other.
12. A heating element assembly as set forth in claim 11 wherein
said upper face of said lower layer is bonded to said lower surface
of said bundle and said marginal portions are bonded in
face-to-face relation to each other by pressure sensitive
adhesive.
13. A heating element assembly as set forth in claim 11 wherein
said upper face of said lower layer is bonded to said lower surface
of said bundle and said marginal portions are bonded in
face-to-face relation to each other by heat activated adhesive.
14. A heating element assembly as set forth in claim 11 wherein
said marginal portions are bonded together by ultrasonic welds.
15. A heating element assembly as set forth in claim 11 wherein
said webs are of equal transverse width.
16. A heating element assembly as set forth in claim 11 wherein
said webs are of unequal transverse width.
17. A heating element assembly as set forth in claim 1 including
coding means for visually distinguishing said bonded lower layer
from said unconnected upper layer.
18. A heating element assembly as set forth in claim 17 wherein
said coding means comprises a color code.
19. A heating element assembly as set forth in claim 1 wherein said
bundle has a terminal end portion projecting axially outwardly
beyond an associated end of said upper layer and said lower layer
has a bundle stabilizing tab projecting axially outwardly beyond
said associated end in underlying relation and bonded to said
terminal end portion.
20. A heating element assembly as set forth in claim 1 wherein the
thickness of said lower layer is greater than the thickness of said
upper layer.
21. A heating element assembly as set forth in claim 1 wherein said
bundle has an electrical resistance in the range of 0.1 to 3.0
ohms/linear foot.
22. A heating element assembly as set forth in claim 1 wherein said
bundle has an electrical resistance in the range of 2 to 3 ohms per
linear foot.
23. A heating element assembly as set forth in claim 1 wherein said
bundle has a thickness to width ratio of approximately one to
twenty-five.
24. A heating element assembly as set forth in claim 1 wherein said
bundle and said dielectric sheath are flexible.
25. A heating element assembly as set forth in claim 1 wherein said
upper and lower layers have substantially the same thickness.
26. A heating element assembly comprising; an axially elongated
flexible carbon fiber tow having a generally flat configuration and
including from 1 thousand to 50 thousand axially elongated
generally cylindrical continuous rectilinear axially extending
carbon filaments having a diameter from 6 to 20 microns and
arranged in immediately adjacent parallel relation to each other,
said tow having an electrical resistance of 2 to 3 ohms per linear
foot, and an outer jacket of polyester sheet material including two
layers of said sheet material arranged in facing relation to each
other with said tow disposed therebetween, one of said two layers
being a substantially flat planar layer, one of said two layers
having a thickness greater than the thickness of the other of said
two layers, said tow adhered to one of said two layers, one of said
two layers overlying said tow in direct contacting engagement and
unconnected relation to said tow.
27. A heating element assembly comprising; a series of axially
elongated axially parallel flexible carbon fiber tows of
undetermined axial length each spaced from another and having
interstacies therebetween, each of said tows including a
multiplicity of continuous generally rectilinear axially parallel
carbon filaments disposed in immediately adjacent relation to each
other and having a predetermined electrical resistance per unit of
tow axial length, and an outer insulating jacket of dielectric
sheet material including a substantially flat planar first layer
and a second layer, said tows adhered to said first layer, said
second layer overlying said tows in direct contacting engagement
with and unconnected relation to said tows and adhered in sealing
relation to said first layer along said interstacies and along
marginal portions of said outer insulating jacket immediately
outboard of the outermost tows in said series.
28. A method of making a heating element assembly comprising the
steps of; continuously advancing an axially elongated first web of
dielectric sheet material in an axial direction, simultaneously
continuously advancing an axially elongate carbon fiber tow in said
axial direction, moisturizing the tow, guiding the tow into axial
alignment and overlying adhering engagement with the advancing
first web, adhering the tow to the advancing first web,
continuously advancing a second web of dielectric sheet material
into overlying relation with marginal portions of the first web and
the tow adhered to the first web, and joining only axially
extending marginal portions of the first and second webs in
face-to-face sealing engagement with each other to form an outer
sheath containing the tow and embracing the tow along its axial
length.
29. A heating element assembly as set forth in claim 26 wherein one
of said two layers is wider than the other of said two layers.
30. A heating element assembly as set forth in claim 26 wherein
said tow has an electrical resistance of 2 to 3 ohms per linear
foot.
31. A heating element assembly as set forth in claim 26 wherein
said carbon filaments have a diameter of substantially 7
microns.
32. A heating element assembly as set forth in claim 26 wherein
said polyester sheet material comprises KAPTON.
33. A heating element assembly as set forth in claim 26 wherein
said tow has a terminal end portion projecting axially outwardly
from said outer jacket and one of said two layers has a tow
stabilizing tab having a width substantially equal to the width of
said tow and projecting axially outwardly from said outer jacket in
underlying relation to said terminal end portion and bonded to said
terminal end portion.
34. A heating element as set forth in claim 33 wherein said tow and
said outer jacket are flexible.
35. A heating element assembly comprising; a flexible generally
flat carbon fiber tow having a multiplicity of continuous generally
rectilinear parallel carbon fibers extending in an axial direction,
said tow having substantially flat upper and lower surfaces
parallel to each other and a predetermined electrical resistance
per unit of axial length, and an axially elongated outer jacket of
dielectric sheet material including two layers of said sheet
material arranged in face-to-face relation to each other with said
tow disposed therebetween, said two layers having marginal portions
projecting outwardly in axially transverse directions from opposite
sides of said tow, said marginal portions being bounded together
and sealed in face-to-face relation to each other and extending in
axial directions along said opposite sides of said tow, one of said
two layers being bonded to one of said surfaces comprising said
upper surface and said lower surface, one of said two layers being
disposed in overlying direct contacting engagement and unconnected
relation to one of said surfaces comprising said upper surface and
said lower surface.
36. A heating element assembly as set forth in claim 35 wherein one
of said two layers is a substantially flat planar layer.
Description
FIELD OF THE INVENTION
This invention relates in general to heating element assemblies and
deals more particularly with improvements in non-metallic heating
element assemblies of electrical resistance type.
BACKGROUND OF THE INVENTION
The present invention is particularly concerned with improvements
in non-metallic heating element assemblies and particularly
electrically conductive carbon fiber heating element assemblies
suitable for general purpose usage in a wide variety of heating
applications.
The development of improved processes for artificially producing
staple carbon in fibrous or filamentary form and at reasonable cost
has virtually revolutionized the plastic composite industry,
particularly where light weight and a high degree of mechanical
integrity is desired. New materials embodying carbon in fibrous or
filamentary form now enjoy wide spread use in the production of
golf shafts, aircraft parts, and indeed entire airframes, to cite a
few outstanding examples. Advantages in the use of carbon in the
electrical field were early recognized by pioneers in that field,
and although artificially produced carbon fiber has found some
limited usage in electrically operated heating device, the
potential for such usage has not yet been fully realized.
Accordingly, it is the general aim of the present invention to
provide improved electrical heating element assemblies employing
carbon fiber technology and suitable for supplying heat in a wide
variety of environments both small and large. It is a further aim
of the present invention to provide improved carbon fiber heating
element assemblies for use in a variety of automotive heating
applications as, for example, warming the seats and steering wheel
of a vehicle, deicing and defogging windows, outside rear view
mirrors and various vehicle engine heating applications.
A still further aim of the invention is to provide improved carbon
fiber heating elements for economic installation and operation to
heat large surface areas such as the floors of up-scale homes,
apartments and condominiums, the surfaces of parking lots,
sidewalks, driveways, highway sections, bridge decks and airport
runways, as well as the surfaces of aircraft which operate
thereon.
Yet another aim of the invention is to provide carbon fiber heating
element assemblies, which utilizes to advantage the negative
coefficient of electrical resistance (ohm) exhibited by carbon
fiber.
SUMMARY OF THE INVENTION
In accordance with the present invention, a heating element
assembly comprises an axially elongated longitudinally extending
generally flat bundle of substantially rectilinear continuous
carbon fibers or filaments of indeterminate axial length. The
bundle has a predetermined electrical resistance per unit of axial
length and is disposed between generally flat layers of dielectric
sheet material arranged in opposing face-to-face relation to each
other with one of the layers in direct overlying contacting
engagement with and unconnected relation to an associated flat
surface of the bundle and the other of the layers adhered to
another flat surface of the bundle opposite the associated flat
surface. Marginal portions of the layers are connected to each
other along the entire axial length of the bundle and immediately
adjacent longitudinally extending transversely opposite sides of
the bundle, whereby a sheath formed by the layers is sealed against
axially transverse migration of moisture through the sheath. The
sheath also serves to electrically insulate the bundle.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a fragmentary perspective view of a heating element
assembly embodying the present invention.
FIG. 2 is a somewhat enlarged sectional view taken along the lines
2--2 of FIG. 1.
FIG. 3 is a fragmentary perspective view showing another embodiment
of the invention.
FIG. 4 is a somewhat enlarged sectional view taken along the lines
4--4 of FIG. 3.
FIG. 5 is a somewhat schematic side elevational view illustrating a
method of making a heating element assembly in accordance with the
present invention.
FIG. 6 is a fragmentary schematic perspective view and illustrates
a method for stripping an end portion of the heating element
assembly of FIGS. 1 and 2.
FIG. 7 is a fragmentary perspective view of the stripped end
portion of the heating element assembly of FIG. 6.
FIG. 8 is a fragmentary perspective view of a further heating
element assembly made in accordance with the invention.
FIG. 9 is a somewhat enlarged fragmentary sectional view taken
along the line 9--9 of FIG. 8.
FIG. 10 is a diagrammatic view of an apparatus for determining the
electrical resistance per unit of length of a typical heating
element assembly embodying the present invention.
FIG. 11 is a diagrammatic view of an apparatus for determining the
electrical resistance per unit of length of a test sample at
various operating temperatures.
FIG. 12 is a graphic illustration of test results obtaining using
the apparatus of FIG. 11.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Turning now to the drawings and referring first particularly to
FIGS. 1 and 2, a typical heating element assembly embodying the
present invention and made in accordance with the invention is
indicated generally by the reference numeral 10. In the description
which follows and in the claims directional terms such as upper and
lower are employed for convenience and refer to the illustrated
heating element assembly 10 as oriented in the drawings, however,
it should be understood that the heating element assembly of the
present invention may be operated in any orientation.
The illustrated heating element assembly 10 essentially comprises
an axially elongated substantially flat bundle of individual
continuous carbon fibers or filaments, which cooperate to form an
electrical heating element which transforms electrical energy
applied thereto into heat energy, the flat bundle or heating
element being designated generally by the reference numeral 12 and
that individual fibers or filaments being indicated at 14, 14. The
assembly 10 further includes an outer jacket or electrically
insulating sheath, indicated generally at 6, formed by lower and
upper layers of relative thin dielectric sheet material 18 and 20,
respectively. The layers 18 and 20 are of equal width and
thickness, and arranged in opposing face-to-face relation to each
other with the heating element 21 disposed therebetween. The upper
face of the lower layer 18 is boded to the lower surface of the
bundle 12, whereas the lower face of the upper layer 20 is disposed
in direct contacting engagement with an in unconnected relation to
the associated upper surface if the flat bundle 12, which it
directly overlies and compliments.
Longitudinally extending marginal portions of the layers 18 and 20,
indicated at 22, 22 and 24, 24, respectively, project outwardly in
opposite axially transverse directions for some distances beyond
the longitudinally extending opposite sides of the bundle 12 and
are joined in face-to-face relation to each other by appropriate
connecting and sealing means along each side of the bundle and
along substantially the entire axial length of the bundle 12 for
preventing migration of moisture transversely through the sheath 16
formed by the connected layers 18 and 20. The connected lower and
upper marginal portions 22, 22 and 24, 24, respectively, may also
serve as mounting flanges for securing the heating element assembly
10 in an operating position relative to associated product or
structure to be heated. In the illustrated embodiment 10 the
connecting and sealing means comprise a coating of pressure
sensitive adhesive, indicated at 26, best shown in FIG. 2, and
initially applied to and carried by the lower layer 18.
In accordance with presently preferred construction, both the
carbon filaments 14, 14, which comprise the heating element or
bundle 12, and the layers of dielectric sheet material from which
the outer sheath 16 is formed are flexible so that the heating
element assembly 10 may be produced in indeterminate length for
storage on a dispensing reel or the like and to facilitate flexure
during mounting and/or when in use, if necessary. Ultimately, the
length of the heating element assembly 10 will be determined by the
particular requirements of the product or structure in which it is
utilized.
Considering now the heating element assembly 10 in further detail,
in accordance with presently preferred construction, the bundle 12
comprises a generally flat carbon fiber tow having a multiplicity
of artificially produced carbon fibers or filaments 14, 14 and a
thickness to width ratio of about 1 to 25. The tow may be made from
polyacrylonitrile (PAN) or other suitable polymer precursor by a
pyrolizing process, as is well known in the carbon fiber art. The
terms carbon fiber tow and carbon filament tow as used herein, and
in the claims, refer to a loose, untwisted, rope-like flat bundle
of continuous generally rectilinear parallel carbon fibers
extending in an axial direction (i.e. slender and greatly elongated
axially extended filaments) which may include from several hundred
individual continuous generally rectilinear flexible filaments 14,
14 to several tens of thousands of such filaments and having an
electrical resistance in the range from 0.1 to 20 ohms per linear
foot. However, in accordance with present practice, a tow having
from 1 thousand to 50 thousand generally cylindrical filaments or
fibers 14, 14 each having a diameter ranging from 6 to 10 microns
and an electrical resistance (cold) in the range of 2 to 3 ohms per
linear foot, plus or minus 0.10 ohm, is used in practicing the
invention, a tow having 50,000 filaments of 7 micron diameter being
presently preferred.
A commercial grade carbon fiber tow, that is a tow which is 94 96
percent pure carbon by weight may be employed in practicing the
invention. A tow of military grade may also be employed. However, a
tow of the later type, which is 98 percent pure carbon by weight,
is considerably more expensive to produce and, for this reason, a
commercial grade material is presently preferred and should result
in a heating element suitable for most heating applications.
The outer jacket or insulating sheath 16 may be made from any
suitable flexible dielectric plastic material. However, since the
heating element assembly 10 is designed to operate within a
temperature range from approximately minus 100.degree. F. to
250.degree. F., the dielectric material chosen for use in making
the sheath 16 must be capable of withstanding temperatures within
the aforesaid anticipated operating range without undergoing an
appreciable change in physical characteristics or a significant
increase in its rate of deterioration. The flexible sheath material
should also possess the required characteristic which allow it to
be bonded to itself or to another material either by a suitable
adhesive or by a non-adhesive bonding process which provides a
moisture-tight seal of substantial integrity in the region of
joinder. As previously noted, a relatively thin plastic sheet
material is used in making the heating element jacket 16, MYLAR, a
thermoplastic polyester, being a presently preferred material. The
sheath may also be made from a polyimide, KAPTON being a preferred
material where a sheath of thermosetting material may be
desired.
The entire jacket or sheath 16 may be made from the same material
as, for example, polyester sheet or web material having a thickness
of two mil (0.0002 inch) (0.0508 millimeter). However, the upper
layer 20 is the preferred heat transfer medium because it is in
direct contact with the heating element 12, unlike the lower layer
18 which is or may be separated from the heating element by a layer
of adhesive which provides some degree of heat insulation. Since
the upper and lower surfaces of the heating element assembly 10
have differing heat transfer characteristics, the assembly is
preferably coded to enable one layer to be readily distinguished
from the other. A color coding is presently preferred wherein the
layers are of differing colors to assure proper mounting and
provide the most efficient heat transfer to an associated surface
or structure to be heated.
In FIGS. 3 and 4 there is shown another heating element assembly
embodying the invention and indicated generally at 10a. Parts of
the assembly 10a which correspond to parts of the previously
described assembly 10 bear the same numerals as the previously
described parts with a letter "a" suffix and will not be further
described in detail.
The assembly 10a differs from the assembly 10 in that it has a
generally flat planar lower layer 18a, which is substantially
thicker than the upper layer 20a. It will also be noted that the
upper layer 20a is made from a web of material substantially wider
than the web from which the lower layer 18a is made. The lower
layer 18a, that is the layer which is connected to and stabilizes
the tow 12a, has a thickness somewhat greater than the thickness of
the upper layer or unconnected layer 20a, which preferably
comprises the heat transfer medium. Thus, in accordance with a
presently preferred construction, the lower layer may, for example,
have a thickness of two mil (0.0002 inch) (0.0508 mm) whereas the
thickness of the upper layer 20 may be 1 mil (0.0001 inch) (0.0254
mm).
The heating element assembly of the present invention, exemplified
by the assembly 10, is preferably produced by a continuous forming
process shown somewhat schematically in FIG. 5 wherein the tow 12,
which has been preferably previously produced with a flattened
cross section configuration, is moisturized and continuously
advanced and guided by a set of guide rolls or other suitable
guiding means into alignment and overlying engagement with the
upper face of a continuously advancing lower layer or web of
polyester sheet material 18, the entire upper face of which is
precoated with a pressure sensitive adhesive 26. A continuously
advancing second web or upper layer of sheet material 20 is
simultaneously guided and fed into overlying engagement with the
advancing tow 12 and the advancing first layer 18 which underlies
the tow. The advancing subassembly, which includes the tow 12, the
adhesive coated lower layer 18 and the uncoated upper layer 20,
passes between a set of pressure rollers, indicated generally at
30, which generally compliment the cross-sectional configuration of
the aforesaid subassembly. The pressure rollers press the marginal
portions 24, 24 of the uncoated upper layer 20 into adhering
engagement with complimentary marginal portions 22, 22 of the
pressure sensitive adhesive coated lower layer 18. Pressure is
simultaneously applied to the central portion of the subassembly to
adhere the lower surface of the flattened tow 12 to the adhesive
coated upper face of the lower layer 18 whereby to complete
formation of the advancing sheath 16, which then embraces the
simultaneously advancing flattened tow 12.
A similar forming process may be employed using a heat-activated
adhesive preapplied to the first or lower layer, for example. The
adhesive may be activated by heated pressure rolls or other
suitable heating mean during the sheath forming process. If a
heat-activated adhesive is employed, an additional curing or drying
cycle may be included in the process to complete assembly of the
sheath 16. Once activated the heat activated adhesive takes a
permanent set and remains substantially unchanged even after
application of additional heat.
Various other bonding processes may be employed to join and seal
the marginal portions of the upper layer 20 to associated marginal
portions of the lower layer 18 and/or to connect the tow 12 to the
lower layer 18. Thus, for example, the marginal portions may be
joined by an ultrasonic welding process or the simultaneous
application of heat and pressure as, for example, where the
marginal portions are passed between heated rollers or the like.
However, any process employed to attach the upper face of the lower
layer to the lower surface of the tow must be capable of effecting
attachment without destroying or otherwise damaging the electrical
continuity of the elongated fibers or filaments which comprise the
tow.
As previously noted, the length of a heating element assembly will
be determined by the particular heating requirements of the product
or structure in which it is to be employed. When the required axial
length of the heating element assembly has been determined,
opposite end portions of the heating element 12 are prepared for
electrical termination. More specifically, and with further
reference to the assembly 10, a portion of the outer jacket or
sheath 16 is removed from each end portion of the heating element
assembly 10 to prepare the heating element 12 for electrical
termination, that is to facilitate electrical connection to an
electrical power source (not shown). Each end portion of the
completed heating element assembly 10 is prepared for electrical
termination by stripping from the assembly 10 an end portion of the
upper layer 20 which overlies the tow 12 and associated marginal
end portions 22, 22 and 24, 24 of the upper and lower layers which
extent transversely outwardly beyond the tow. Stripping is best
accomplished using an electrically heated nickel chromium wire
under tension, shown at 31 in FIG. 6. The heated wire 31 is pressed
downwardly on the assembly 10 to cut entirely through both upper
and marginal portions and through the upper layer 20 down to the
upper surface of the tow 12. Since the melting temperature of the
sheath 16 is much lower than that of the carbon fiber tow 12, the
hot wire stripping operation may be preformed without risk of
damaging the tow. Secondary slits, indicated at 32, 32 in FIG. 6,
are cut or otherwise formed at opposite sides of the tow 12 and in
parallel relation to the direction axial extent of the tow whereby
the central end portion of the upper layer 20 and the entire
associated marginal end portions formed by the joinder of the lower
and upper layers are removed. The resulting stripped terminal end
portion of the heating element assembly 10, indicated generally at
34 in FIG. 7, includes an end portion of the tow 12 which extends
outwardly beyond the end of the upper layer 20 and an extending tab
35 formed by a portion of the lower layer 18. The tab 35 underlies
the extending terminal end portion 34 and is adhered to the lower
surface of the terminal end portion. Thus, the bare upper surface
of the tow end portion 34 is exposed beyond the cut end of the
upper layer 20 to facilitate electrical termination, whereas the
extending lower portion or tab 34 on the lower layer 18 remains
connected to the lower surface of the tow to stabilize the tow
terminal end portion 34. Thus, the relatively fine, delicate
exposed end portion of the tow 12 derives support from the
extending tab 35 on the lower layer 20 which is adhered to
filaments at 14, 14 at the lower surface of the tow so that the
resulting exposed terminal end portion 34 can be conveniently
handled during a later electrical termination process without
substantial risk of damage to the tow.
Further referring to the drawings and particularly FIGS. 8 and 9,
another heating element assembly embodying the present invention is
indicated generally at 10b. The illustrated heating element
assembly 10b essentially comprises a series of axially elongated,
axially parallel flexible carbon fiber tows 12b, 12b of
undetermined axial length. The tows may vary in number and are
spaced apart and have interstacies 36, 36, therebetween. Each tow
12b has a multiplicity of continuous axially parallel carbon
filaments 14b, 14b disposed in immediate adjacent relation to each
other and a predetermined electrical resistance per unit of tow
length. The tows 12b, 12b are sandwiched between opposing first and
second layers of polyester sheet material, preferably MYLAR,
indicated at 18b and 20b, respectively. The layers 18b and 20b are
bonded together in face-to-face relation to each other along the
interstacies 36, 36 and along longitudinally extending marginal
portions 22b, 22b, located outboard of the outermost tows in the
series to form a dielectric outer jacket 16b. The marginal bonds
between the layers 18b and 20b have a high degree of sealing
integrity to prevent transverse migration of moisture into the
jacket 16b through the marginal portions thereof and between the
tows within the jacket 16b. The face of one of the layers 18b and
20b is bonded to associated surfaces of the tows 12b, 12b to
stabilize the tows as hereinbefore discussed. The other of the
layers is disposed in direct contacting engagement with the tows
12b, 12b, but is not connected to the tows, which allows the
assembly 10b to be cut to desired length at any point along its
entire length. The resulting end portion may be stripped at that
time or at a later time for electrical termination, as hereinbefore
discussed. The unconnected layer preferably serves as a heat
transfer medium when the heating element assembly 10b is mounted in
a device or structure to be heated.
Heating element assemblies in accordance with the invention are
adapted to operate within a temperature range, which utilizes to
advantage the negative temperature coefficient characteristic of
carbon fiber. Thus, when a heating element assembly of the present
invention is operated within such a temperature range, 150.degree.
F. to 200.degree. F., for example, the electrical resistance of the
heating element assembly decreases as the temperature of the
heating element increases. The advantage attained by utilizing the
aforesaid phenomenon will be better understood from a comparison of
a typical carbon fiber heating element assembly and one of a
conventional metal type.
Referring now to FIG. 10, a test apparatus for determining the
electrical resistance of the aforesaid heating element network at
standard temperature and pressure is illustrated and indicated
generally at 40. A carbon fiber tow or heating element 12 of the
type used in making the heating element assembly 10 and having 50
thousand carbon fibers of seven micron diameter and a 10 foot
length is electrically terminated at 42, 42 by lead conductors 44,
44 suitable for interconnection to electrical instrumentation. A
four wire bridge type ohm meter 46 is used to measure the total
resistance of the ten-foot network at standard temperature and
pressure, whereby the resistance per foot of axial length is
determined to be 2.8 ohms/ft.
The aforedescribed network is then connected to another testing
apparatus which includes a variable DC voltage source 48 (0 50
VDC), an amp meter 50 and a thermometer 52, as shown as in FIG. 11.
The voltage is slowly adjusted until the temperature of the carbon
fiber tow reaches 150.degree. F. Thereafter, the voltage is
increased to produce 10.degree. F. increments of temperature
increase until the temperature of the network reaches 200.degree.
F. The voltage (volts) and amperage (amps) for each 10.degree.
incremental increase in temperature is recorded. The electrical
resistance for each 10.degree. temperature increment is then
calculated by applying ohm's law.
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A conventional metal heating element of 10 foot length is
substituted for the carbon fiber network and the aforesaid test and
calculations are repeated and the results are recorded for the
metal heating element sample. The accumulated data is then used to
plot the graphic illustration shown in FIG. 12. It will be noted
that the resulting curve 54 for the carbon fiber heating element 12
has a negative slope throughout the anticipated operating range,
which is characteristic of carbon fiber material, whereas the
comparable curve 56 for a metallic heating element exhibits a
positive slope throughout the entire anticipated operating range,
indicating that electrical resistance increases as the temperature
of the heating element material increases when operated within the
range under consideration.
The aforesaid data will allow a designer to implement a carbon
fiber heating element system to achieve a desired criteria. A
typical heating application employing the aforesaid data developed
for a heating element assembly 10 which has a 50 thousand (50K)
carbon fiber tow and is designed to operate at 160.degree. F. will
now be considered.
Employing the data developed for a 50 K carbon fiber heating
element assembly 10 where an element temperature of 160.degree. F.
is desired: R @ standard temperature and pressure=2.8 ohms/ft. R @
160.degree. F.=2.2 ohms/ft I @ 160.degree. F.=1.5 amps V=IR
V=1.5.times.2.2=3.3 volts DC/ft Applying the data developed for a
metal heating element operating at 1.5 amps where R @ 160.degree.
F.=3 ohms/ft V=1.5.times.3=4.5 volts/foot DC
An increase in voltage input per foot of 1.2 volts or 27% is
required by the metal heating element.
The development of similar data should enable a designer to
implement a carbon fiber heating element assembly to achieve a
desired criteria.
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